Polyphase filter

ABSTRACT

A polyphase filter ( 20; 100 ) is described, having two filter channels ( 30   I   , 30   Q   ; 101   I   , 101   Q ) for processing an I-input signal (φ I ) and a Q-input signal (φ Q ) which is shifted over 90° with respect to the I-input signal (φ I ), respectively. The filter ( 20; 100 ) has at least two capacitive filter components (C I , C Q ; Ci I , Ci Q ) corresponding to each other in the two filter channels ( 30   I   , 30   Q   ; 101   I   , 101   Q ), wherein the capacity values (C; Ci) of these two capacitive filter components (C I , C Q ; Ci I , Ci Q ) are substantially equal to each other. Said two capacitive filter components (C I , C Q ; Ci I , Ci Q ) are coupled to each other by means of two current source couplings ( 40   QI   , 40   IQ   ; 106   i ) switched in anti-parallel, having substantially equal characteristic. Hereby, a displacement of the filter characteristic toward higher frequencies is achieved, over a distance ω C .

[0001] The present invention relates in general to a polyphase bandpassfilter.

[0002] Such filters are known per se, for instance from the U.S. Pat.No. 4,914,408, and they can for instance be applied in receiver circuitsfor, e.g. radio applications, television applications, or telephoneapplications. Although such filters also know different applications, apossible application of such a filter will be explained here in moredetail in the context of a receiver circuit.

[0003] An important drawback of the filter described in U.S. Pat. No.4,914,408 is that a coupling between two filter channels is effected bymeans of resistors. An important aim of the present invention istherefore to provide a polyphase bandpass filter wherein such couplingresistors are omitted.

[0004] These and other aspects, characteristics and advantages of thepresent invention will be explained in more detail by the followingdescription of a preferred embodiment of a polyphase bandpass filteraccording to the invention with reference to the drawing, in which samereference numerals indicate same or similar parts, and in which:

[0005]FIG. 1 schematically shows a known receiver circuit;

[0006]FIG. 2A schematically illustrates the transmission characteristicof a low pass filter;

[0007]FIG. 2B schematically illustrates the transmission characteristicof a bandpass filter, derived from the transmission characteristic ofFIG. 2A;

[0008]FIG. 2C schematically shows a parallel connection of a capacitorand a complex component;

[0009]FIG. 2D schematically illustrates a known way of coupling twofilter channels;

[0010]FIG. 3A illustrates the coupling principle according to thepresent invention;

[0011]FIG. 3B illustrates a replacement representation of the couplingschedule of FIG. 3A; and

[0012]FIG. 4 is a basic schedule of a embodiment of a polyphase filteraccording to the present invention.

[0013]FIG. 1 schematically shows a known receiver circuit 1, in which areceiver signal S coming from an antenna 2 is mixed in a first mixingstage 3 with a signal φ provided by a local oscillator 5, and whereinsaid signal S is mixed in a second mixing stage 4 with a second signalprovided by the local oscillator 5 which is 90° shifted with respect tothe first signal φ. The output signal of the first mixing stage 3, whichis also indicated by the phrase inphase signal, is fed to a first input11 of a filter 10, and the output signal of the second mixing stage 4,which is also indicated by the phase quadrature signal, is fed to asecond input 12 of the filter 10. The filter 10 has two filter channels13 and 14, respectively, which process the inphase signal of the firstinput 11 and the quadrature signal of the second input 12, respectively,in substantially identical way, and which have outputs 15 and 16,respectively, for providing an inphase output signal and a quadratureoutput signal, respectively, wherein the quadrature output signal of thesecond output is shifted 90° with respect to the inphase output signalof the first output. The filter channels 13 and 14 have mutual identicalfilter characteristics, for example a bandpass characteristic.

[0014] In the following, the frequency of the local oscillator signalwill be indicated by f₁. In the receiver signal S, many frequencies arepresent; in the following, the tuning frequency to which the receptioncircuit 1 must be tuned will be indicated by f₂. Assume that thisfrequency is higher than the local frequency f₁, i.e. that f₂=f₁+Δf.

[0015] In general, when two signals having two different frequencies f₁and f₂ are mixed, a signal component results with the differencefrequency Δf−f₂−f₁. However, in the receiver signal S there will also bea frequency f₃ present, fulfilling f₃=f₁−Δf. This component alsocontributes to the eventual mixing signal, i.e. by causing a virtualmirror signal component with the difference frequency f₃−f₁=−Δf. It isdesired that the filter 10 suppresses such mirror signal components.

[0016] Further it is desired that the filter 10 has a bandpasscharacteristic which is substantially symmetrical with respect to thecenter frequency ω_(C).

[0017] A known way of designing a bandpass filter of which the frequencycharacteristic is symmetrical with respect to a center frequencyω_(C)>0, and wherein mirror signal components axe effectivelysuppressed, starts with a lowpass filter of which the characteristiccorresponds to the desired characteristic of the bandpass filter to bedesigned.

[0018]FIG. 2A schematically illustrates the transfer characteristic of alowpass filter. Along the horizontal axis, the frequency ω is set out,and the transfer function H is set out along the vertical axis. Inpractice, only signals can occur of which the frequency is higher thanzero; this part of the frequency characteristic is shown with a solidline. The frequency characteristic, however, does not depend on the signof the frequency, which implies that the frequency characteristic issymmetrical with respect to ω=0, as is shown in FIG. 2A with a dottedline.

[0019] Depending on the design of the lowpass filter, said lowpassfilter can have a desired characteristic, for instance first order,second order, or higher order, Bessel-type, Butterworth-type, etc.Starting from the lowpass filter with the desired characteristic, abandpass filter can be derived by a transformation or shifting of thefilter characteristics to a higher frequency. FIG. 2B shows thecharacteristic of FIG. 2A, shifted over a distance ω_(C) to a higherfrequency. The transfer function H_(BDF)(ω) of this bandpass filterfulfills the following formula:

H _(BDF)(ω)=H _(LDF)(ω−ω_(C))   (1)

[0020] The desired shift of the filter characteristic corresponds to ashift of all poles and all zeros of the filter over mutually identicaldistances along the imaginary axis. In a filter design of which thecomponents with frequency-depending impedance are capacitiesexclusively, this can be achieved by switching a complex component X inparallel to said capacitive filter components, of which the admittanceY_(X) is a constant complex number according to the formula

Y _(X)(ω)=−j·ω _(C) ·C   (2)

[0021]FIG. 2C schematically shows a parallel connection of a capacitor Cand such a complex component X. For the frequency-depending admittanceY_(C) of a capacitor with a capacitive value C, the following formulaapplies in the case of an ideal capacitor

Y _(C)(ω)=j·ω·C   (3)

[0022] For the frequency-depending admittance Y of the parallelconnection of FIG. 2C, the following is valid:

Y(ω)=Y _(C)(ω)+Y _(X)(ω)=j·ω·C−j·ω _(C) ·C=j·(ω−ω _(C))·C   (4)

[0023] In the case of a signal with frequency ω, the behaviour of thisparallel circuit is, therefore, identical to the behaviour of thecapacitor C at a frequency ω−ω_(C). By replacing all capacitive filtercomponents of a filter by such parallel circuits, the behaviour of theoverall filter at a frequency ω will therefore be identical to thebehaviour of the original filter at a frequency ω−ω_(C).

[0024] The above deduction already applies for a single filter. Aproblem when realizing a bandpass filter in this way, then, is moved toa problem of providing a component of which the admittance (orinversely: the impedance) is a constant complex number. Although this ispossible per se, in a polyphase filter it is possible to use in anelegant way the fact that there are two mutually identical filterchannels present, in which the signals are mutually identical butshifted over 90° with respect to each other. Then, in such a filter, thebehaviour of the complex component X can be obtained by using, in eachchannel, a component of which the admittance is a real number, but whichreceives at its input the 90° shifted signal from the other channel.

[0025] Such an approach for the construction of a polyphase bandpassfilter has already been described in the U.S. Pat. No. 4,914,408. There,the real coupling between two filter channels is effected by means ofresistors, as illustrated in FIG. 2D. Therein, each resistor has aresistance value R=ω_(C)·C.

[0026] This known approach, however, has some objections, which areparticularly relevant when realizing the filter on a chip. Caused byprocess variations, the resistors and capacitors of the filter will showa relatively large tolerance. Therefore, the capacitors and resistorsshould be able to be set after manufacture. However, this is difficultto realize.

[0027] It is a general goal of the present invention to overcome thementioned disadvantages.

[0028] More particularly, the present invention aims to provide apolyphase bandpass filter wherein the coupling between two filterchannels, necessary for achieving the desired frequency shift, iseffected without resistors.

[0029] According to an important aspect of the present invention, thecoupling between two filter channels is exfected by means of avoltage-controlled current source. This principle in accordance with thepresent invention is illustrated in FIG. 3A. In FIG. 3A, a polyphasefilter is generally indicated by the reference numeral 20. The filter 20has two mutually identical filter channels 30, which will be indicatedwith the index I and Q, respectively, for distinction with respect toeach other. Each filter channel 30 _(I), 30 _(Q) has an input 31 _(I),31 _(Q) and an output 32 _(I), 32 _(Q). Since the design of the filterchannels 30 _(I), 30 _(Q) can be any suitable design, while variousconstructions for filter channels are known per se, the complete designof the filter channels 30 is not shown in FIG. 3A.

[0030] For the sake of the following discussion, one capacitive filtercomponent C_(I) of the inphase filter channel 30 _(i) is shown in FIG.3A, and the corresponding capacitive filter component C_(Q) of thequadrature filter channel 30 _(I) is shown. The two capacitive filtercomponents C_(I) and C_(Q) are coupled with each other by means of twocurrent source couplings 40 _(QI) and 40 _(IQ) connected inanti-parallel. The first current source coupling 40 _(QI) comprises afirst voltage-controlled current source 41 _(I) of which the output isconnected in parallel to the capacitive filter component C_(I) in theinphase filter channel 30 _(I), while the second current source coupling40 _(IQ) comprises a second voltage-controlled current source 41 _(Q) ofwhich the output is connected in parallel with the correspondingcapacitive filter component C_(Q) in the quadrature filter channel 30_(Q).

[0031] The first voltage-controlled current source 41 _(I) is controlledby an output signal of a first voltage detector 42 _(Q), of which theinput is connected in parallel with the capacitive filter componentC_(Q). Similarly, the second voltage-controlled current source 41 _(Q)is under control of a second voltage detector 42 _(I) of which the inputis connected in parallel with the capacitive filter component C_(I).

[0032] Thus, the first voltage-controlled current source 41 _(I) adds tothe first filter channel 30 _(I) a current of which the value depends onthe voltage over the capacitive filter component C_(Q) in the secondfilter channel 30 _(Q), while the second voltage-controlled currentsource 41 _(Q) adds to the second filter channel 30 _(Q) a current ofwhich the value depends on the voltage over the capacitive filtercomponent C_(I) in the first filter channel 30 _(I).

[0033] The two current source coupling 40 _(QI) and 40 _(IQ) can bemutually identical, although this is not necessary. Important is only,that the proportionality factors between the voltage detected by thevoltage detector 42 and the current generated by the current source 41are mutually identical for both current source coupling 40 _(QI) and 40_(IQ); in other words: important is only that the two current sourcecouplings 40 _(QI) and 40 _(IQ) have mutually identical transfercharacteristics. This implies that each current source coupling 40 _(QI)and 40 _(IQ) is designed for letting the voltage-controlled currentsource 41 _(I) and 41 _(Q), respectively, generate a current I_(41,I),and I_(41,Q), respectively, of which the current magnitude depends onthe voltage V_(CQ) and V_(CI), respectively, detected by the voltagedetector 42 _(Q) and 42 _(I), respectively, according to the formulas

I _(41,I) =V _(CQ)/(ω_(C) ·C)

[0034] and

I _(41,Q) =V _(CI)/(ω_(C) ·C)

[0035] wherein C is the capacitive value of the two capacitive filtercomponents C_(I) and C_(Q), respectively, and wherein ω_(C) is thedesired center frequency of the bandpass filter.

[0036] In the following, a combination of two current source couplingsconnected in anti-parallel will be indicated with the phrase “gyrator”,and will be indicated by the symbol 50 shown in FIG. 3B. A gyrator 50has two terminals 51A and 51B. For coupling from 51A to 51B, the gyrator50 comprises a first current source coupling not shown in FIG. 3B, ofwhich terinal 51A is a voltage input and of which terminal 51B is acurrent output. For coupling from 51B to 51A, the gyrator 50 comprises asecond current source coupling not shown in FIG. 3B, of which terminal51B is a voltage input and of which terminal 51A is a current output.The two current source couplings each have a proportionality factorG_(AB) and G_(BA), respectively, defined as output current divided byinput voltage. When both proportionality factors are equal to eachother, or at least have an identical characteristic, the gyrator will beindicated as a symmetrical gyrator. This can be achieved if both currentsource couplings are identical, but this is not necessary.

[0037]FIG. 4 shows an example of an implementation of a polyphase filter100 according to the present invention. The polyphase filter 100comprises an inphase channel 101 _(I) and a quadrature channel 101 _(Q),which are mutually substantially identical. The channels 101 _(I), 101_(Q) have inputs 102 _(I), 102 _(Q) for receiving an inphase inputsignal φ_(I) and a quadrature input signal φ_(Q), respectively. Thechannels 101 _(I), 101 _(Q) further have outputs 103 _(I), 103 _(Q) foroutputting an inphase output signal ψ_(I) and a quadrature output signalψ_(Q), respectively. The inputs 102 _(I), 102 _(Q) are current inputs,i.e. the input signals ψ_(I) and ψ_(Q) are current signals; if it isdesired that the filter 100 receives voltage signals, voltage-to-currentconverters can be switched before the inputs 102 _(I), 102 _(Q); sinceknown per se voltage-to-current converters can be used for this, theywill not be described in more detail here. The outputs 103 _(I), 103_(Q) are voltage outputs, i.e. the output signals ψ_(I) and ψ_(Q) arevoltage signals; when it is desired that the filter 100 outputs currentsignals, voltage-to-current converters can be switched after the outputs103 _(I), 103 _(Q); since known per se voltage-to-current converters canbe used for this, these will also not be described in more detail here.

[0038] The channels 101 _(I), 101 _(Q) comprise a plurality of Ncapacities C1 _(I), C2 _(I), C3 _(I), . . . CN_(I) and C1 _(Q), C2 _(Q),C3 _(Q), . . . CN_(Q), respectively, wherein N≧2.

[0039] In the inphase channel 101 _(I), two subsequent capacities Ci_(I)and C[i+1]_(I) are always coupled by a gyrator 105 i _(I). Similarly, inthe quadrature channel 101 _(Q), two subsequent capacities Ci_(Q) andC[i+1]_(Q) are always coupled by a gyrator 105 i _(Q). The correspondinggyrators 105 i _(I) and 105 i _(Q) are mutually identical; herein, the“forward” proportionality factor G(i→i+1) and the “backwards”proportionality factor G(i+1→i) need not be mutually identical.

[0040] The corresponding capacities Ci_(I) and Ci_(Q) always havemutually identical capacity values Ci; for different values of i, thecapacity values Ci can be different. The corresponding capacities Ci_(I)and Ci_(Q) are always coupled to each other by a symmetrical gyrator 106i; the proportionality factors Gi_(IQ) and Gi_(QI) of each gyrator 106 iare always equal to 1/(ω_(C)·Ci).

[0041] Thus, the present invention provides a polyphase filter 20; 100with to filter channels 30 _(I), 30 _(Q); 101 _(I), 101 _(Q) forprocessing an I-input signal φ_(I) and a Q-input signal φ_(Q),respectively. The filter has at least two capacitive filter componentsC_(I), C_(Q); Ci_(I), Ci_(Q) corresponding to each other in the twofilter channels 30 _(I), 30 _(Q); 101 _(I), 101 _(Q), wherein thecapacity values C; Ci of these two capacitive filter components C_(I),C_(Q); Ci_(I), Ci_(Q) are substantially equal to each other. Said twocapacitive filter components C_(I), C_(Q); Ci_(I), Ci_(Q) are coupled toeach other by means of two current source couplings 40 _(QI), 40 _(IQ);106 i with substantially equal characteristic, switched anti-parallel.Hereby, a displacement of the filter characteristic over a distanceω_(C) toward higher frequencies is achieved.

[0042] It will be evident to a person skilled in the art that the scopeof the present invention is not limited to the examples discussed in theabove, but that several amendments and modifications thereof arepossible without deviating from the scope of the invention as defined inthe attached claims.

1. Polyphase filter (20; 100) comprising: a first filter channel (30_(I); 101 _(I)) with a desired filter characteristic, with an input (31_(I); 102 _(I)) for receiving an I-input signal (φ_(I)) and an output(32 _(I); 103 _(I)) for providing an I-output signal (ψ_(I)); a secondfilter channel (30 _(Q); 101 _(Q)), substantially identical to the firstfilter channel (30 _(I); 101 _(I)), with an input (31 _(Q); 102 _(Q))for receiving a Q-input signal (φ_(Q)) which is shifted 90° with respectto the I-input signal (φ_(I)), and an output (32 _(Q); 103 _(Q)) forproviding a Q-output signal (ψ_(Q)) which is shifted 90° with respect tothe I-output signal (ψ_(I)); said filter (20; 100) having at least onecapacitive filter component (C_(I); Ci_(I)) in the first filter channel(30 _(I); 101 _(I)) and a capacitive filter component (C_(Q); Ci_(Q))corresponding therewith in the second filter channel (30 _(Q); 101_(Q)), wherein the capacity values (C; Ci) of these two capacitivefilter components (C_(I), C_(Q); Ci_(I), Ci_(Q)) are substantially equalto each other; wherein said two capacitive filter components (C_(I),C_(Q); Ci_(I), Ci_(Q)) are coupled to each other by means of two currentsource couplings (40 _(QI), 40 _(IQ); 106 i) with substantially equalcharacteristic connected in anti-parallel.
 2. Filter according to claim1 , wherein each current source coupling (40 _(QI), 40 _(IQ)) comprisesa voltage-controlled current source (41 _(I), 41 _(Q)) with a voltageinput and a current output, wherein the current output is coupled inparallel with a capacitive filter component (C_(I), C_(Q)) of the onefilter channel (30 _(I), 30 _(Q)) while the voltage input is coupled inparallel to the corresponding capacitive filter component (C_(Q), C_(I))of the other channel (30 _(Q), 30 _(I)).
 3. Filter according to claim 1or 2 , wherein a first current source coupling (40 _(QI)) comprises: afirst voltage detector (42 _(Q)) of which the input is switched inparallel with said corresponding capacitive filter component (C_(Q)) inthe second filter channel (30 _(Q)); a first voltage-controlled currentsource (41 _(I)) which is switched in parallel with said capacitivefilter component (C_(I)) in the first filter channel (30 _(I)), andwhich is controlled by an output signal of the first voltage detector(42 _(Q)); wherein a second current source coupling (40 _(IQ))comprises: a second voltage detector (42 _(I)) of which the input isswitched in parallel with said capacitive filter component (C_(I)) inthe first filter channel (30 _(I)); a second voltage-controlled currentsource (41 _(Q)) which is switched in parallel with said correspondingcapacitive filter component (C_(Q)) in the second filter channel (30_(Q)), and which is controlled by an output signal of the second voltagedetector (42 _(I)); and wherein each current source (41 _(I), 41 _(Q))is adapted to provide a current (I_(41,I), I_(41,Q)) of which thecurrent magnitude fulfills I _(41,I) =V _(CQ)/(ω_(C) ·C) respectively I_(41,Q) =V _(CI)/(ω_(C) ·C).
 4. Filter according to any of the previousclaims, wherein each filter channel (101 _(I), 101 _(Q)) comprises atleast one combination of two capacities (Ci_(I) and C[i+1]_(I), Ci_(Q)and C[i+1]_(Q)) which are coupled to each other by a gyrator (105 i_(I), 105 i _(Q)).
 5. Filter according to claim 4 , wherein of saidcombinations, the first capacity (Ci_(I)) of the first filter channel(101 _(I)) and the first capacity (Ci_(Q)) of the second filter channel(101 _(Q)) corresponding therewith, as well as the second capacity(C[i+1]_(I)) of the first filter channel (101 _(I)) and the secondcapacity (C[i+1]_(Q)) of the second filter channel (101 _(Q))corresponding therewith, are mutually coupled to each other by means ofalways a symmetrical gyrator (106 _(I), 106[i+1]) of which theproportionality factors (Gi_(IQ) and Gi_(QI), G[i+1]_(IQ) andG[i+1]_(QI)) are always equal to 1/(ω_(C)·Ci) and 1/(ω_(C)·C[i+1]),respectively.
 6. Filter according to any of the previous claims, whereineach filter channel (30 _(I), 30 _(Q); 101 _(I), 101 _(Q)) has aplurality of capacitive filter components, and wherein each filtercomponent in the first filter channel (30 _(I); 101 _(I)) is coupled, bymeans of two current source couplings (40 _(QI), 40 _(IQ); 106 i)switched anti-parallel, to the capacitive filter component (C_(Q),Ci_(Q)) corresponding therewith in the second filter channel (30 _(Q);101 _(Q)).
 7. Filter according to any of the previous claims, whereineach individual filter channel (30 _(I), 30 _(Q); 101 _(I), 101 _(Q))has a lowpass filter characteristic, and wherein the polyphase filter(20; 100) has a bandpass filter characteristic caused by said currentsource couplings (40 _(QI), 40 _(IQ); 106 i).